The technical field, to which the invention relates, concerns the production of tissue paper on a corresponding paper making machine, in which in particular a through air drying (TAD) zone is present. A special impressing tissue is used in this TAD zone.
The sheet formation of the paper and the three-dimensional structuring of a moist non-woven fabric which has already been formed but can still be deformed due to a high residual water content is normally carried out on supporting fabrics which originate from textile weaving processes.
The three-dimensional structure of a moist paper sheet by the formation of zones of lesser density, which are framed by compressed zones, is carried out in modern tissue producing machines within the framework of pre-drying of the sheet in a pre-drying part before the Yankee cylinder. The pre-drying of the paper sheet is carried out on the supporting fabric by convection, in that hot air is pressed through the paper sheet which is located on the supporting fabric.
One speaks of xe2x80x9cthrough air dryingxe2x80x9d or TAD.
The three-dimensional structuring is conventionally carried out in three steps, which usually succeed each other while being locally separated. The first step is formed by a deflection of the fibres in the Z-direction into the structuring recesses of the supporting fabric which are provided by the TAD impressing fabric which are systematically distributed over the area of the fabric which is contacted by paper. The deflection of the fibres in the Z-direction is caused by air and water flows, supported by a vacuum in one or, in certain circumstances, a plurality of suction boxes which is/are arranged on the side of the supporting fabric opposite the side contacted by the paper.
The deflection of the fibres in the Z-direction into the interior of the recesses creates in the paper sheet zones of reduced density, which are also described as pillows. These zones of reduced density which are arranged in a pattern are dried in a second step on and/in the interior of the supporting fabric by the air which passes through one or a plurality of TAD cylinders and thereby it is fixed in the present fibre distribution. We speak of a xe2x80x9cfreezingxe2x80x9d of the state of fibre distribution.
In a third step, partial compression is then carried out of the pre-dried non-woven fibre fleece by pressing the supporting fabric with the pre-dried sheet laid on it, by means of a press roller against the surface of the Yankee cylinder. The compression of the paper sheet is carried out on the raised points of the supporting fabric which can be formed both in certain zones of the surface of the supporting fabric by warp wires as well as by weft wires. Thereby the fibres which are located in recesses of the fabric remain protected against compression. TAD imprinting fabrics, as a supporting fabric, constitute a special form of the fabric which has its typical structure-forming properties due to the type of weaving, the yarn selection with respect to material, diameter, cross-sectional form and later treatment, for example the thermo-fixing and grinding of the surface.
Paper making machine fabrics are known for example from WO 96/04418, DE OS 30 08 344, EP 0 724 038 A1.
The technical problem (object) of the invention is to create a paper making machine fabric which is suitable and designed with respect to a tissue paper which is produced thereby of an improved three-dimensional surface structure in the form of a sequence of recesses and elevations for the achievement of a tissue paper with improved appearance, improved softness and increased volume in connection with an improved water absorption and an improved feel.
This problem is solved, in particular, by the features of patent claim 1.
This solution is based on the finding that in the fabric of the paper machine fabric in the weft and warp yarns of the fabric at selected points warp yarns are superimposed on three successive intersections of weft yarns and weft yarns are superimposed on three successive intersections of warp yarns, so that on the top of the paper making machine fabric, i.e. on the side of the fabric which contacts the paper, longer sections of the warp or weft yarns are exposed and by comparison with a normal standard weave had a larger degree of freedom of movement upwards, i.e. perpendicularly to the plane of the fabric. This greater degree of freedom of movement results during the matching of such yarn sections in the formation of relatively deep recesses, which provide the paper making machine fabric with an increased total height and therefore with higher three-dimensionality. This is increased, when in the zone of the at least three overlays the outer intersection points in direct succession are designed as based intersection points, at which the yarn is raised with the greater degree of freedom, when in the weaving process, the fabric is compressed after the introduction of the weft yarn and, in particular, a relatively dense fabric is produced in which the weft and warp yarns preferably no longer extend in a straight line in the transverse and longitudinal directions. This effect can also appear and, in certain circumstances, more strongly in connection with the dense fabric, if a thermo-shrinkage is caused in a relatively loose fabric. The forces which arise in this context lead to a strong raising of the yarn with a greater degree of freedom. Therefore due to the solution in accordance with the invention a paper making machine fabric is created, in which outstandingly deep recesses are present with the result that, in particular, in the TAD zone by using this paper making machine fabric a paper and, in particular, a tissue paper can be produced which has outstandingly great three-dimensionality with respect to increasing volume, which permits the paper to seem to be especially fluffy, and, in addition, shows outstanding capacity for water absorption together with excellent softness. In addition, the result is an improved similarity to a woven structure and therefore a character which is more similar to cloth.
When there is mention of a number of types of weft yarns, this means that such a number of weft yarns is present which take a different direction.
Special further embodiments with respect to the tissue paper result from subclaims 7 to 28. Accordingly the paper has two different types of non-compressed zones of different form or size. The non-compressed zones are surrounded by compressed zones, so-called picket zones. The different types of non-compressed zones can have a different number of creping folds. Each type of non-compressed zone always has preferably the same number of creping folds. It can occur that individual non-compressed zones of a certain type have a number of creping folds which do not deviate from the ideal value. But smaller non-compressed zones can also be present without creping folds and the larger non-compressed zones can be present with two creping folds.
Due to this product there is a substantially improved feel in that the mechanical properties of the tissue paper are substantially altered and/or improved. The textile type character is also substantially strengthened. There is also an improvement with respect to liquid absorption and adsorption by relatively large pockets to store liquid and large surface pores to accept particles. The tissue paper has a clearly greater thickness.
Further embodiments follow from the sub-claims. A further increase in the depth of the recesses can be achieved by the features of claim 2. A series of examples of embodiments emerges from the other sub-claims.